Spectroscopic analysis of data from nuclear well logging operations can provide quantitative or qualitative information related to a geological formation surrounding a borehole. In the context of formation evaluation, the term formation refers to the volume of rock seen by a measurement made in the borehole, as in a log or a well test. These measurements indicate the physical properties of this volume.
Data on downhole conditions and movement of the drilling assembly can be collected during or after the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize well placement. Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are commonly referred to as measurement-while-drilling (“MWD”). Similar techniques focusing more on the measurement of formation parameters during the drilling process are commonly referred to as logging-while-drilling (“LWD”). However, the terms MWD and LWD are often used interchangeably, and the use of either term in this disclosure will be understood to include both the collection of formation and borehole information as well as data on movement and placement of the drilling assembly. Wireline logging tools can be conveyed in a wellbore by a wireline cable, drill pipe, coiled tubing, tractor, or other suitable tool conveyance means.
For example, the formation may scatter radiation artificially introduced into it by a nuclear well logging tool. Specifically, the nuclear well logging tool can include a neutron source that bombards the formation with neutrons, which interact with nuclei in the formation to produce gamma-ray radiation. The tool also includes one or more gamma-ray scintillation detectors that measure the amount of gamma-ray radiation from the formation and its energy spectrum as a function of time. The tool can also include one or more neutron detectors that measure the amount of thermal neutron radiation from the formation and its energy spectrum as a function of time. These measurements can give quantitative information that characterizes petrophysical properties of the formation (such as formation or bulk density, formation porosity, sigma or macroscopic thermal neutron capture cross-section of the formation, lithography, and water saturation).
Furthermore, the formation itself may contain materials that emit radiation naturally. In this case, the gamma-ray scintillation detector of the tool can be used to measure the spectrum of nuclear radiation (e.g., gamma-rays) emitted naturally by the formation. The measured spectrum (or background spectrum) can be used in order to discern petrophysical properties of the formation. The background spectrum can also be subtracted from the measured spectrum resulting from neutron bombardment of the formation in order to remove the effects of the naturally emitted gamma-rays from the measurement.
The gamma-ray scintillation detector of the nuclear well logging tool includes a scintillator coupled to an electronic light sensor such as a photomultiplier tube (PMT), photodiode, or silicon photomultiplier. A scintillator is a material that exhibits scintillation—the property of luminescence, when excited by an incident gamma-ray radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, (i.e. re-emit part of the absorbed energy in the form of light). Sometimes, the excited state is metastable, so the relaxation back down from the excited state to lower states is delayed (necessitating anywhere from a few nanoseconds to hours depending on the material). The process then corresponds to either one of two phenomena, depending on the type of transition and hence the wavelength of the emitted optical photon: delayed fluorescence or phosphorescence, also called after-glow.
A PMT absorbs the light emitted by the scintillator and re-emits it in the form of electrons via the photoelectric effect. The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator. Thus, when gamma-ray radiation from the formation strikes the gamma-ray scintillation detector, the gamma-ray scintillation detector may generate an electrical signal corresponding to the energy of the incident gamma-ray radiation.
There are various properties of scintillator materials which may be beneficial to their use in gamma-ray scintillation detectors, such as high density, fast operation speed, low cost, radiation hardness, production capability and durability of operational parameters.
High density scintillator materials can reduce the size of the scintillation detector for high-energy gamma-rays. The range of Compton scattered photons for lower energy gamma-rays is also decreased with the use of high density scintillator materials. This results in high segmentation of the detector and leads to better spatial resolution. High density scintillator materials can also have heavy ions in the lattice (e.g., lead, cadmium), which can significantly increase the photo-fraction. The increased photo-fraction is relevant for some applications, such as positron emission tomography (PET). High stopping power for the electromagnetic component of the ionizing radiation needs greater photo-fraction, which may allow for a more compact detector.
Operating speed can affect the scintillators resolution of spectra. For example, high operating speed may be beneficial for improving resolution of spectra due to the avoidance of “pile-up”. The energy measured in a single pulse becomes inaccurate if it piles up on the tail of a previous pulse. Therefore, it is preferable that, on average, the scintillation pulse decays before another pulse arrives. In addition, the signals may be time tagged, making a fast response preferable. For example, for certain types of measurements (e.g., “Sigma”), data is acquired in time histograms with time bins as short as 1 microsecond. Therefore, for example, it is preferable that detectors measuring Sigma have reasonably short response times of less than 1 microsecond.
Several other properties are also desirable for good scintillator material, including a high efficiency for converting the energy of incident radiation into scintillation photons, transparency to its own scintillation light (for good light collection), efficient detection of the radiation being studied, a high stopping power, good linearity over a wide range of energy, a short rise time for fast timing applications (e.g., coincidence measurements), a short decay time to reduce detector dead-time and accommodate high event rates, emission in a spectral range matching the spectral sensitivity of existing PMTs (although wavelength shifters can sometimes be used), and an index of refraction near that of glass (≈1.5) or sapphire to allow optimum coupling to the PMT window.
Among the properties listed above, the light output can affect both the efficiency and the resolution of the gamma-ray scintillation detector. The energy resolution is defined as the ratio of the full width, at half maximum, of a given energy peak to the peak position, usually expressed as a percentage. The light output is a strong function of the type of incident particle or photon and of its energy, which therefore strongly influences the type of scintillation material to be used for a particular application. The light output is often quantified as a number of scintillation photons produced per MeV of deposited energy. Typical numbers are (when the incident particle is an electron): ≈40,000 photons/MeV for NaI(Tl), ˜10,000 photons/MeV for plastic scintillators, and 8,000 photons/MeV for bismuth germanate (BGO). The presence of quenching effects results in reduced light output (i.e., reduced scintillation efficiency). Quenching refers to all radiationless de-excitation processes in which the excitation is degraded mainly to heat.
Various electronic neutron sources are used in logging tools for neutron bombardment of the formation, but are typically limited in strength due to constraints in size and power consumption. To compensate for limited neutron source strength available in logging tools noted above, the gamma-ray scintillation detectors of the nuclear logging tools for oilfield applications can benefit from having high detection efficiency. Detection efficiency of a detector can depend on different parameters. In that regard, various definitions of efficiency are used in the literature to relate some of those different parameters: (i) absolute efficiency is the ratio of the number of counts recorded by the gamma-ray scintillation detector to the number of gamma-rays emitted by the source (in all directions); (ii) intrinsic efficiency is the ratio of the number of pulses recorded by the gamma-ray scintillation detector to the number of gamma-rays hitting the gamma-ray scintillation detector; (iii) full-energy peak (or photopeak) efficiency is the efficiency for producing full-energy peak pulses only, rather than a pulse of any size, for the gamma-ray. As space within an oilfield measurement tool is limited, a detector package may also be limited in size (e.g., depending on application, approximately 13 to 76 mm diameter and 13 to 200 mm long), which may make achieving high detection efficiency more challenging.
The overall signal production efficiency of the gamma-ray scintillation detector, however, also depends in part on the quantum efficiency of the PMT (typically about 30% at peak for room temperature PMTs and about 20% for high temperature PMTs), and on the efficiency of light transmission and collection (which depends on the type of reflector material covering the scintillator and light guides, the length/shape of the light guides, any light absorption, etc.).
Further, other considerations for performance of the gamma-ray scintillation detector include the operating temperatures and vibration tolerance. The coupled PMTs also exhibit temperature sensitivity, and can be damaged if submitted to mechanical shock. In the case of oilfield use, a scintillator detector may experience a range of operating temperatures (e.g., −40° C. to 175° C.), and high-vibration or shock. For many scintillators, light output and scintillation decay time depend on environmental temperature experienced by the scintillator. This dependence is often ignored for room-temperature applications (i.e., in a laboratory environment) since it is usually small over a small temperature range around room temperature.
The above-noted characteristics of scintillator materials and the range of downhole environmental conditions in which the gamma-ray scintillation detectors may operate have resulted in few choices of gamma-ray scintillation detectors.
Tl-activated Sodium Iodide (NaI(Tl)) has been used as a scintillator material in the gamma-ray scintillation detectors for some nuclear logging tools. The scintillation decay of NaI(Tl) is reasonably fast with a primary time constant of about 230 ns at room temperature (25° C.). Also, the light yield of NaI(Tl) is also reasonably tolerant to temperature increases, making NaI(Tl) usable in a downhole environment to temperatures up to 175° C., and even up to 200° C. for certain applications.
Ce-activated Gadolinium Oxyorthosilicate (GSO) and self-activated Bismuth Germanate (BGO) have been used as scintillator materials in gamma-ray scintillation detectors for some nuclear logging tools. Both GSO and BGO are mechanically stronger and chemically more stable than NaI(Tl). In addition GSO and BGO are more efficient than NaI(Tl) for gamma-ray detection due to their higher density and higher effective atomic number (Zeff). However, it is common for high atomic number materials other than NaI(Tl), such as GSO and BGO, to still exhibit low light yield even at room temperature [Kob2007] and/or for their light output to drop significantly with increasing temperature [Boa2013]. For example, BGO [Boa2013], bismuth silicate (BSO) [Kob1983], and lead tungstate (PWO or PbWO4) [Kob2007] each has a room temperature light yield of less than 1500 ph/MeV [Kob2007], except BGO which exhibits about 6,000 ph/MeV at room temperature [Kob2007]. These are the highest room temperature light yield values of high atomic number materials in the literature. However, BGO experiences rapidly decreasing light yield as temperature increases. For example, at 70° C., BGO's light yield is one-half of its light yield at room temperature.
The above-mentioned decrease in light output at higher temperatures causes the energy resolution to decrease at higher temperatures, which, in-turn reduces the signal-to-noise ratio and increases statistical uncertainty in the radiation measurements obtained by the scintillation detector. Thus, such temperature-sensitive scintillator materials, such as GSO and BGO, are deemed to be unsuitable (without auxiliary cooling of the scintillator detector) for use in scintillator detectors intended for use at temperatures above 150° C. for an extended time. Temperature limitations also apply to commercially available solid state gamma detectors (e.g. High Purity Germanium (HPGe) or Cadmium Zinc Telluride (CZT)), which are not used downhole.
Recently, Ce-activated Lanthanum Bromide (LaBr3) and Ce-activated Yttrium Aluminum Perovskite (YAP) have been used as scintillator materials in gamma-ray scintillation detectors for some nuclear logging tools, particularly for high count rate applications due to their decay times of less than 30 nanoseconds. However, Ce-doped LaBr3, is extremely sensitive to contaminants, even inside a hermetically sealed detector, and the sensitivity is exacerbated even more when the scintillator operates at elevated temperatures (i.e., above 70° C.).
While it is possible to employ work-arounds to avoid the aforementioned temperature effects on scintillator detectors, workarounds may be inconvenient and costly. For example, one workaround is to passively cool a scintillation detector by placing the detector in a Dewar flask. However, such cooling is time-limited and adds to cost and complexity of the tool.
Also, the light emission spectra of many commercial scintillator materials that are used under laboratory conditions are not favorable for use with PMTs typically used in oilfield scintillation detectors, which are mostly sensitive in the deep violet and ultra-violet (UV) range. Such commercial scintillator materials may thus require other PMTs, which can increase the complexity and cost of the scintillation detector.